feat: Add 'convert_local_content' method to directly convert file content (str)

[Athroniaeth]

Hello, maybe I didn't find it, but I couldn't figure out how to directly convert the content of a file (str) into markdown. This PR contains the unit tests for this method

from markitdown import MarkItDown

md = MarkItDown()
result = md.convert_local_content("<h1>Hello World!</h1>", file_extension=".html")
print(result.text_content)

-->

# Hello World!

[Athroniaeth]

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